1. Field of the Invention
The present invention relates generally to a corrosion detecting probe for detecting corrosion of steel materials buried in existing armored concrete structures and more particularly concerns a transportable corrosion detecting probe which is capable of quantitatively determining the state of corrosion through an electrochemical measurement.
2. Description of the Related Art
The steel materials employed in the armored or reinforced concrete structures such as buildings are subjected to corrosion under various conditions and undergo degradation in the function. Under the circumstances, it is a common practice at present to examine or measure the degree of corrosion of the steel material and determine the use life of the reinforced concrete structure on the basis of the result of the measurement. To this end, there is generally adopted a method of inspecting the steel material according to which a concerned structure of armored concrete is partially destroyed to expose a reinforcing steel member for performing visual inspection thereof. However, this type of inspection is obviously unfavorable for the structure or building itself.
As an attempt to evade the problem mentioned above, there has been proposed a method of determining or measuring nondestructively the corrosion of steel material in the state buried or embedded in the concrete structure, as is exemplified by the one disclosed in Japanese Patent Application Laid Open No. JP-A-59-217147. According to this prior art method, a steel bar embedded in a concrete structure is partially exposed to be used as a working electrode to which a test electrode terminal is connected. On the other hand, a transportable electrode device including a combination of a reference electrode and a counter electrode both immersed in a liquid electrolyte solution is moved sequentially over the concrete surface in intimate or close contact therewith along a path where the steel bar to be inspected is embedded, wherein the rest potential, polarization resistance and solution resistance are measured with the aid of the abovementioned reference electrode, working electrode and the counter electrode. On the basis of these three electrochemical characteristic quantities as measured, the state or level of corrosion of the steel material is estimated.
In this connection, a method of measuring effectively the corrosion susceptibility of a local portion of an elongated metal member in an electrolyte solution without need for coating the member with an insulation material except for the local portion of concern is disclosed in Japanese Patent Publication No. 61-29457. Additionally, an application of this method to a multi-point measurement is disclosed in Japanese Patent Publication No. 61-33380.
The prior known corrosion detecting techniques mentioned above however suffer from difficulties described below when they are to be used for the nondestructive measurement of the steel material embedded in the concrete structure.
In the first place, description will be made of the corrosion measuring method proposed in JP-A-59-217147 by reference to FIGS. 2 and 3 of the accompanying drawings. According to this method, a steel bar 8 embedded in a concrete structure is used as a working electrode and attached with a working electrode terminal 9 at a portion exposed only partially, while a transportable electrode device 1 including a combination of a reference electrode 4 and a counter electrode 5 both immersed in a liquid electrolyte solution 3 is placed on the surface of the concrete structure in close contact therewith sequentially over a number of locations along the embedded length of the steel bar 8, wherein at each of the above-mentioned locations, the rest potential, polarization resistance and the solution resistance are measured by means of the reference electrode 4, the working electrode 9 and the counter electrode 5. On the basis of the three electrochemical characteristic values thus obtained, the state or level of corrosion is estimated. This method is certainly advantageous in that the steel member embedded in concrete can be measured nondestructively. FIG. 2 shows graphically distributions of current flow lines 31 and equipotential lines 32 which make appearance when a certain voltage is externally applied between the counter electrode 5 and the working electrode 9, which certain voltage is determined with reference to the rest potential (i.e. the potential making appearance across the reference electrode 4 and the working electrode 9 in the state where no external potential is applied between the counter electrode 4 and the working electrode 9. As will be seen in FIG. 2, the current flowing to the steel member 8 embedded in the concrete structure from the counter electrode 5 disposed within the transportable electrode device 1 propagates extensively, while the range within which the current flow takes place differs remarkably in dependence on the thickness h of the concrete layer 7, giving rise to problems. For determining the influence brought about by the factors mentioned above, simulation is performed on an electrically conductive paper sheet of a size comparable to that of an actual concrete surface to be tested. The results of the simulation are graphically illustrated in FIG. 3, in which the ratio of current density i/i.sub.0 in the steel member embedded in concrete (where i.sub.0 represents the current density (in A/cm.sup.2) measured in the steel portion located immediately underneath the transportable electrode device 1) is taken along the ordinate with the abscissa representing the distance L, from the point underlying immediately below the transportable electrode device. Comparison of the two curves shows clearly that the current distribution represented by the curve marked with a series of x for the case where h=30 mm differs remarkably from the current distribution represented by the curve attached with small circles for the case where h=50 mm. Needless to say, the electrochemical characteristic quantities such as the polarization resistance R.sub.p (.OMEGA. cm.sup.2) and the solution resistance R.sub.s (.OMEGA. cm.sup.2) are only meaningful when they can be defined on the basis of the current flow per unit area. It will however be noted that in the case of the method described above, the surface region of the steel bar 8 which exerts influence to the measurement can not be limited only to a small area to be measured. As a consequence, the polarization resistance and the solution resistance measured at various locations on the concrete surface are influenced by the thickness h of the concrete coating, thus making it meaningless to compare simply the measured resistance values.
Next, reference is made to the methods disclosed in Japanese Patent Publications Nos. 61-29457 and 61-33380. Both of these methods teach that when a local portion of an elongated metal member is to be measured in respect to the corrosion susceptibility within a liquid electrolyte solution, measurement can be performed with high efficiency without coating the metal member except for the local portion to be subjected to the direct measurement. More specifically, these known methods are advantageous in that the electrochemical measurement can be accomplished on the basis of the current measured only at the local surface region of the steel member of concern by virtue of such electrode structure that a probe electrode (hereinafter referred to as the counter electrode) is enclosed by a guard electrode (hereinafter referred to as the current flow line control electrode). However, these prior art methods are incapable of conducting the corrosion measurement nondestructively for the steel member embedded in the concrete structure for the reasons elucidated below. First, the method disclosed in Japanese Patent Publication No. 61-29457 suffers from two problems:
(1) It is impossible to ensure electrically preferable conduction in a nondestructive manner between a steel material of concern embedded in a concrete structure and the concerned electrode of the detecting probe.
(2) Any accurate measurement of potential is rendered impractical with the reference electrode disposed in the vicinity of the corrosion detecting probe which is composed of a counter electrode and a current flow line control electrode.
Since the problem (1) is self-explanatory, any further discussion will be unnecessary. Accordingly, following discussion is directed to the problem (2).
Assuming that the problem (1) is solved by some suitable measures and that the method disclosed in Japanese Patent Publication No. 61-29457 is to be applied for evaluation of the corrosion of a steel member buried in a concrete structure, then disposition of the individual electrodes upon actual measurement will be such as shown in FIG. 4 of the accompanying drawings. More specifically, the corrosion detecting probe 13 composed of the counter electrode 5 and the current flow line control electrode 11 is disposed at a position 16a above the steel member 8 with a distance of 20 mm to 50 mm, as is in the case of the conventional measurement.
In that case, the problem to be first pointed out is seen in that the rest potential of the steel member in the corroded state which usually differs in dependence on the position of measurement can not be measured at the point 16b on the concrete surface located nearest to the surface point 16a of the steel member of concern, but the potential of a value approximating that of the rest potential appearing at the point 16c on the surface of the steel member of concern will be measured, because the position of the surface point 16a of the steel member 8 which is really the subject for measurement is deviated from the location 16d where the reference electrode 4 is disposed.
Even when the first problem mentioned above is solved by realizing the corrosion detecting probe 13 in as small a size as possible or by any other appropriate means, to thereby minimize error involved in the measurement of the rest potential, there still arises another problem in conjunction with the electrochemical measurement (e.g. measurement of polarization or AC impedance) carried out by applying a certain voltage determined with reference to the rest potential of the steel member 8 between the latter and the counter electrode 5, which is electrically connected to the current flow line control electrode 11, to thereby produce current flows thereacross.
First, the problem intervening the polarization measurement will be considered. As will be seen from a potential/current distribution map depicted in FIG. 4 on the basis of the results of analysis of the actual measurement, equipotential lines are much concentrated below the current flow line control electrode 11. As a consequence, the potential difference measured between the steel member 8 and the reference electrode 4 which is located in the vicinity of the current flow line control electrode 11 will vary significantly even when the point 16d where the reference electrode 4 is positioned is deviated only slightly. Magnitude of such change in the measured potential will become more significant as the voltage applied across the steel member 8 and the electrodes 5, 11 is increased. This holds true even in case the reference electrode 4 can be stationally secured with a high accuracy at a predetermined distance relative to the corrosion detecting probe 13 by some suitable means, because the thickness h of the concrete layer is not usually uniform in strict sense but varies more or less in dependence on the position at which the corrosion detecting probe 13 is disposed, whereby the location within the body of concrete to which the measured potential value bears relevancy (the location 16e in the case of the illustrated example) can not be determined definitely. As the result, for the polarization measurement in which correction or compensation is required for ohmic voltage drop (IR loss of potential), extremely complicate procedure is involved because correction must be made on the ohmic voltage measured between the locations 16a and 16e upon every measurement after the potential and current distributions have been analyzed in detail for identifying accurately the location (16e) where the potential is detected. This problem becomes more serious in the measurement in which a system of high resistance such as concrete intervenes.
Next, the problem associated with the measurement of AC impedance will be considered. In the case of the AC impedance measurement, influence of the ohmic drop of potential does not present any noticeable problem, but there arises instead a problem that the current distribution varies remarkably in dependence on the frequency employed in the AC impedance measurement, which will be elucidated below. FIG. 5 of the accompanying drawings illustrates an experimental simulation of the AC impedance measurement performed by using an electrically conductive paper sheet. In the figure, a reference numeral 18 denotes generally an electrode unit composed of two electrodes of the corrosion detecting probe, 17 denotes a concrete structure, 19 denote a plurality of divided steel surface segments, 20 and 21 denote capacitors and resistors each connected in parallel for representing a corrosion equivalent circuit, 14 denotes a non-resistance type ampere meter, and a reference numeral 23 denotes an AC power supply source. In this experiment, values of the currents flowing through the steel segments 19 were measured by using AC currents of two different frequencies f (e.g. f=10 Hz and 1 KHz). The results obtained in the measurement are graphically illustrated in FIG. 6, in which logarithmic value of the current i (mA) per segment is taken along the ordinate with the segment identification number being taken along the abscissa. As will be seen in FIG. 6, the currents flowing through the individual segments vary as a function of the frequency, which in turn means that the voltage and current distributions illustrated in FIG. 4 also undergo variation as a function of the frequency employed in the measurement. More specifically, since the essential feature of the corrosion measurement by the AC impedance method resides in that the current flow between the steel member 8 buried in concrete and the two electrodes (i.e. the counter electrode 5 and the current flow line control electrode 11) incorporated in the corrosion detecting probe is controlled with reference to the potential at the location 16d (same potential as that at the location 16e) where the reference electrode is positioned, the actual voltage applied across the steel member and the electrodes not only can be established definitely but also undergoes variation in dependence on the frequency employed in the measurement. As the result, the impedance value determined arithmetically by using a conventional electrochemical measuring/analysis apparatus 15 is necessarily poor in accuracy.
Finally, the problem which will occur when the method disclosed in Japanese Patent Publication No. 33380/1986 is applied to evaluation of corrosion of the steel material embedded in the concrete structure will be touched briefly. This known method shares the basic concept with the one disclosed in Japanese Patent Publication No. 61-29457 and allows a multi-point measurement to be performed with a plurality of counter electrodes disposed as the current control electrodes. Accordingly, the method under consideration naturally suffers from the same problem as that of the method disclosed in the last mentioned publication. Besides another problem arises additionally. Because the multi-point measurement is performed by using the reference electrode disposed at a given position, the result of the multi-point measurement will be of no use for evaluation of corrosion for such species of object in which the rest potential exhibiting different values in dependence on the location is of great importance for evaluation of the corrosion (such as, for example, for the evaluation of corrosion of the steel member in the longitudinal direction).
As will now be understood from the foregoing discussion, the aforementioned problem (2) provides a great obstacle in the electrochemical measurement of steel member embedded in a concrete structure as compared with the measurement within the electrolyte solution where the position of the reference electrode can be selected arbitrarily.